1. Field of the Invention
The present invention relates to a metal inert gas (MIG) welded joint between aluminum and steel members and a MIG welding process, and in particular to a MIG welding technique used for welding an overlapped region which is made by overlapping planar portions of dissimilar materials, namely, the aluminum and steel members.
2. Description of the Related Art
In recent years, from the viewpoint of protecting the global environment and saving energy, there is required restriction of emissions of harmful gases and carbon dioxide emitted by automobiles, and also required improvement of mileage. In order to respond to such requirements, reduction in weight of vehicle body is the most effective to meet the requirement, so that there are actively studied replacements of steel members with aluminum members, as members to form body parts and other various parts. However, in terms of production cost, it is difficult to convert all the members of vehicle body parts and components into aluminum members. Thus, when the member formed of aluminum is used, it is inevitable to perform dissimilar metal welding between aluminum and steel, namely, hybrid welding, which has been a major concern. In addition, the hybrid welding between aluminum and steel is required not only in the field of transportation vehicles represented by the above-mentioned automobiles, but also in other various fields relating to home electric appliances, construction materials such as building materials, and the like. In those fields, strong welding is an essential requirement.
Therefore, there have been studied various processes of joining together an aluminum member and a steel member, which include the following mechanical connection, such as connection by caulking or using a riveting or a bolt. However, each of these processes inherently has problems in terms of an efficiency of the joining operation, a reliability of the joint, and a cost of the joint, to a greater or less extent.
In addition, considerable improvement in productivity can be expected by employing a conventional fusion welding process such as arc welding commonly used for welding metal members. However, when aluminum and steel members are welded by such a fusion welding process, heat input at the time of welding becomes excessively high and a metallurgical reaction occurs between the molten aluminum and steel members. Thereby, at a welded interface between those members, a brittle and hard intermetallic compound (such as Fe2Al5 or FeAl3) is thickly formed. In consequence, the portion where the compound is formed suffers cracks, so that the applicable joint strength to a practical use cannot be obtained. As can be seen from the above, it has been extremely difficult to perform melt welding between aluminum and steel members.
In a patent document 1 (JP-A-2003-211270), a patent document 2 (JP-A-2003-33865), a patent document 3 (JP-A-2004-223548), a patent document 4 (JP-A-2006-88174), a patent document 5 (JP-A-2006-116599), a patent document 6 (JP-A-2006-224145) and a patent document 7 (JP-A-2006-224147), there are disclosed various processes for welding dissimilar metals of aluminum and steel members by using a MIG welding process, which is a kind of arc welding. However, in the patent document 1, since aluminum and steel members are directly welded by a MIG brazing process using a wire made of copper alloy or nickel alloy, welding cost is increased and a sufficient welding strength is unlikely to be obtained. Therefore, there is still a room for improvement. In the patent document 2, as a filler metal, a flux cored wire is employed in which the flux containing at least caesium fluoride, aluminum fluoride, potassium fluoride, and aluminum oxide as components is coated with aluminum member, thereby suppressing formation of a brittle and thick intermetallic compound layer. However, since the flux remains as slag on a surface of a weld zone, an operation to remove the slag, which covers the surface thereof, is necessary. Thus, problems such as productivity deterioration and an increase in product cost are inherent in the process disclosed in the patent document 2.
In the patent documents 3 to 7, as a filler wire, instead of a brazing filler metal or a flux cored wire, there is employed a solid wire made of aluminum alloy. However, without any particular consideration about a thickness relationship between aluminum and steel members, aluminum and steel members having the same thickness are welded, or a steel member thicker than an aluminum member is welded therewith. Accordingly, even if control is provided to reduce heat input, due to a great difference between rigidities of both members, in other words, due to greater rigidity or stiffness of the steel member than that of the aluminum member (in the case of the same thickness, a rigidity ratio of steel member to aluminum member is about 3 to 1), stress by thermal distortion concentrates when solidification shrinkage occurs after a welding process, whereby local deformation occurs at a weld zone. Accordingly, there is a problem that a defective portion is generated and thus a sufficient joint strength cannot be secured.
In the patent documents 3 to 7, the employed steel members are surface-coated with zinc or zinc-alloy and aluminum or aluminum alloy. There is no disclosure about a process for performing MIG welding between a bare steel member and an aluminum member. Therefore, there has been a desire for a welding process that can form a sound weld zone between aluminum and steel members and secure a sufficient joint strength regardless of the presence or absence of a surface-coated layer or a surface treated layer on a steel member surface.